Candidate stem cells have been isolated from a variety of adult tissues and have previously been thought to be capable of differentiating exclusively into cell lineages belonging to their tissue of origin. Recently, a number of laboratories have provided data which indicates that such tissue-specific stem cells may have broader differentiation programs than originally appreciated and that they may also have the potential to differentiate into cell populations which are beyond their tissues of origin. Several investigators, for example, have reported that bone marrow cells as well as highly purified populations of pluripotent hematopoietic stem cells (PHSC) are capable of producing myogenic cells that play a role in muscle regeneration. Furthermore, skeletal muscle cells have been shown to be not only capable of contributing to studies have led to the development of new paradigms of stem cell commitment and differentiation. Whether such apparent stem cell plasticity is the consequence of the persistence of reservoirs of PHSC within some non-hematopoietic tissues or is the result of the nuclear reprogramming of stem cells belonging to a certain lineage remains unresolved. The overwhelming number of studies of stem cell plasticity has been performed in adult rodents. We hypothesize the existence of a population of primitive stem cells within adult skeletal muscle which has the ability to differentiate not only into myogenic but also hematopoietic cells. In this proposal, the origins of muscle stem cell "trans-differentiation" will be further explored in primates since the behavior of stem cells of small and large animals are known to differ. In addition, the marrow repopulation potential of skeletal muscle stem cells will be examined in a clinically relevant model in order to assess the potential use of skeletal muscle as an alternative to hematopoietic stem cell grafts. In order to achieve these goals, the following specific aims will be pursued: 1) Develop novel in vitro culture conditions which permit the preferential proliferation of muscle stem cells with hematopoietic potential (MSC-HP). 2) Isolate, purify, and characterize human MSC-HP to hematologically reconstitute lethally irradiated non-human primates. Information gained from the completion of these studies will assess the existence of a primitive stem cell within skeletal muscle which can differentiate into both skeletal and hematopoietic progenitor cells and provide a platform for further advancing our understanding of the biology and therapeutic potential of MSC-HP.